This invention relates to apparatus for detection of small amounts of selected vapors or gases in an atmosphere of air or other gas. The invention combines the features of an ionization cell and a drift tube, attaining advantages of both and producing a result which is superior to that produced by either one of them by itself.
The design and operating problems of ionization cells are set forth in U.S. Pat. No. 3,835,328. Such cells can be used to detect the presence of small concentrations of a selected group of chemical vapors or gases in the air or in other vapors or gas backgrounds. U.S. Pat. No. 3,835,328 describes an ionization detector in which a gas sample flows past a source of ionization radiation and through a recombination region to a ion collection region. In the ion collection region a current is produced as a function of the ion concentration in the gas sample. By providing a sufficiently long path and exposure to adequate surface, the recombination of ions is enhanced and controlled in such a way that ion concentration remaining in the gas or vapor sample when it reaches the collection region is a function of trace gases or vapors in the sample. A characteristic of the ionization cell is that the signal is dependent on both the stability of the ions which are formed and upon their concentration. Thus, the cell responds to some interfering vapors or gases as well as to the vapors or gases of interest, even though the size and mobility of interfering ions may be considerably different. An improved ionization cell is disclosed in U.S. patent application Ser. No. 754,148, filed Dec. 27, 1976.
In a drift tube, vapors or gases are subjected to ionizing radiation in the same manner as in the ionization cell. In a drift tube, however, the resulting ions are placed in an electric field, causing the ions to migrate in a predetermined direction, where different types of ions can be separated, detected, and measured by virtue of the difference of velocity or mobility of the ions in an electric field. Ion shutter grids or gates are provided for segregating the ions in accordance with their drift time.